Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under a multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as OFDM signal.
FIG. 5 illustrates the relative timing of the signals from different base stations as observed by a client terminal. A client terminal may observe signals from base stations that may be using different CP types as illustrated in FIG. 5.
Most wireless communication systems may employ some form of framing in the air interface. For example, 10 ms radio frames are used in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system and each radio frame comprises 10 subframes as shown in FIG. 6. Each subframe in turn consists of two slots and each slot consists of 6 or 7 OFDM symbols depending on the type of CP used as shown in FIG. 6. In the 3GPP LTE wireless communication system, two different CP lengths are used and they are referred to as Normal CP and Extended CP. In wireless communication systems, normally the specific air interface frame structure repeats itself over certain periodicity.
The 3GPP LTE wireless communication system uses the following synchronization signals to assist the client terminal in achieving time and frequency synchronization as well as the detection of Physical layer Cell Identity (PCI):                Primary Synchronization Signal (PSS)        Secondary Synchronization Signal (SSS)        
The positions of the PSS and SSS within a frame are illustrated in FIG. 7 for FDD air-interface of a 3GPP LTE wireless communication system. Note that the figure shows the position of the PSS and SSS for both the Normal CP and Extended CP. FIG. 8 illustrates the PSS and SSS positions for TDD air-interface of 3GPP LTE wireless communication system. The PSS and SSS for different cells may be different as described below.
The different PSS and SSS are identified by different signal sequences used for transmission. Specifically, 504 PCI's are defined in 3GPP LTE wireless communication system specifications and they are organized into 168 groups with three identities in each group. The SSS sequence identifies the PCI group and PSS identifies the PCI within a group. Detecting a PCI requires the detection of both the PSS and the SSS.
The PSS sequence in frequency domain is a length 63 Zadoff-Chu sequence extended with five zeros on each side and mapped to central 72 sub-carriers as shown in FIG. 9. The Direct Current (d.c.) subcarrier is not used. In 3GPP LTE wireless communication system three different PSS sequences are used with Zadoff-Chu root indices 24, 29 and 34 corresponding to cell identity 0, 1 and 2 respectively within the PCI group. The exact PSS sequences are defined in the 3GPP LTE specification TS 36.211 “Physical Channels and Modulation,” Jan. 6, 2016, incorporated by reference herein. The time domain PSS signal may be obtained by performing Inverse Discrete Fourier Transform (IDFT) of the frequency domain PSS. The two time domain PSS instances present within a 10 ms radio frame as shown in FIG. 7 and FIG. 8 are identical.
The SSS sequences in frequency domain are generated by frequency interlacing of two length-31 M-sequences X and Y, each of which may take 31 different M-values. The SSS is extended with five zeros on each side and mapped to central 72 sub-carriers as shown in FIG. 10. The d.c. subcarrier is not used. In 3GPP LTE wireless communication system, 168 valid combinations of X and Y are defined corresponding to 168 different PCI groups. The particular values of X and Y are referred herein with m0 and m1. For each of the m0 and m1, there are 31 different values possible. Therefore, the 168 valid PCI groups include m0 and m1 combinations that may have one of the two sequences common with other PCI groups. The time domain SSS signal may be obtained by performing IDFT of the frequency domain SSS. The two SSS sequences present in a 10 ms radio frame are different, namely SSS1 and SSS2 as shown in FIG. 7 and FIG. 8, which allows the client terminal to detect 10 ms radio frame timing from reception of a single SSS. As shown in FIG. 10, the only difference between SSS1 and SSS2 is that the two M-sequences X and Y used in SSS1 are swapped in SSS2. Whether an SSS1 or SSS2 is detected may be indicated by a flag indicating SSS occurrence. The exact relative timing between SSS and PSS depends upon CP type and duplexing type as shown in FIG. 7 and FIG. 8.
At the end of PSS search, OFDM symbol synchronization may be achieved. Therefore, frequency domain processing may be employed for further analysis, such as SSS detection.
The SSS search has to handle unknown timing and frequency offsets in addition to other system unknowns such as CP type and duplexing type. The relative timing (in terms of number of samples) between SSS and PSS varies depending upon CP and duplexing type. Multiple SSS search attempts may be required to resolve unknown system parameters such as CP type and duplexing type. If CP type is known prior to SSS detection, for example using CP correlator, corresponding SSS detection attempt may be skipped. The PSS detection may result in multiple possible PSS positions being detected due to the presence of multiple cells surrounding the client terminal as illustrated in FIG. 5.
In a conventional SSS detection procedure, along with the true cell identification, a high number of false cells may be detected due in part to the search attempts over multiple hypotheses. A cell is said to be a false cell when a cell with the detected PCI is not actually present and yet the PCI is detected during the SSS search procedure. The detection of false cells requires the client terminal to measure and keep track of the qualifying metrics of the false cells in addition to the metrics of the true cells and has to make cell reselection/handover decisions based on the metrics of cells that do not actually exist. This may lead to unnecessary increase in power consumption. Furthermore, if the false cells are present in the detected cell list from SSS processing, unnecessary cell reselections/handover may occur which may reduce the network efficiency and client terminal performance.